The invention relates to a method for producing homopolymers or copolymers of propylene as generically defined by the preamble to claim 1.
One such method is known from German Patent DE 196 45 947 C1.
In this known method, which in turn is based on a method of European Patent Disclosure EP 38 478 B1, homo- or copolymers of the propylene are produced in the gas phase by providing that gaseous or liquid monomers are introduced into a polymerization zone and polymerized; the pressure and the temperature in the polymerization zone are kept within a range appropriate to the gaseous state of the monomers. Gaseous monomer not consumed by polymerization is discharged from the polymerization zone, liquefied, and re-introduced in liquid form into the polymerization zone. The temperature regulation is performed by means of continuous measurement of the temperature and by means of a change, tripped upon a temperature change, in the quantity of liquid monomers introduced into the polymerization zone per unit of time.
The object of the invention was to embody the known method in such a way that it can be performed with still better success; in particular, the constancy of the properties of the polymer over time is to be improved, and reactor yields are to be further increased.
To attain this object, it is proposed that the temperature in the polymerization zone be measured at a plurality of points offset vertically from one another, and that from the various measured values a respective mean value be formed, that the mean values be compared with computed set-point value specifications and that on the basis of the magnitude and sign of the ascertained difference, the quantity of monomers to be fed into the polymerization zone be changed.
By this means, a largely uniform reaction temperature in the agitated powder bed of the polymerization zone can be achieved. It is especially advantageous if for the temperature measurement a polymer discharge tube of the kind schematically shown in FIG. 3 is used. The polymer discharge tube has a plurality of temperature sensors disposed vertically one after the other and is dipped into the powder bed at a point where the motion of the fine-particle, agitated polymer is especially intensive.
In a further characteristic of the invention, the pressure in the polymerization zone is kept constant with the aid of a compensation container for the condensed monomers carried in circulation. In this way, the level of the circular propylene condenser can be regulated as a function of the reactor pressure, something that is not assured with regulating devices alone.
In a further preferred embodiment of the method of the invention, to the polymer discharged from the polymerization zone, propylene and/or mixtures of C2-C8 hydrocarbons are added by polymerization in a second polymerization zone, at pressures of 10 to 30 bar and temperatures of 30 to 90xc2x0 C., in order to alter or improve the physical properties of the polymer.